In one proposal for incorporating optical fibers into cables, individual fibers are coated with polymer, assembled in parallel coplanar groups, and formed into multifiber ribbons; and the ribbons are then assembled one over the other to form a fiber array. The multifiber ribbons are desirably made by laminating the groups of parallel coplanar fibers between two pressure-sensitive adhesive tapes, disposed face-to-face, with the fibers encapsulated in the adhesive between the tape backings.
Polyethylene-terephthalate-backed pressure-sensitive adhesive tapes are a likely choice for forming the multifiber ribbons because they offer a desired combination of mechanical and thermal properties and are low in cost (certain films that would otherwise be useful, such as films of polyimide, polytetrafluoroethylene, and polyparabanic acid, are too costly for widespread use in an optical transmission cable). However, existing polyethlyene terephthalate films have a serious deficiency, in that they lack the extreme resistance to lengthwise or transverse heat-shrinkage required in optical fiber cables. Shrinkage of the tapes, either while the multifiber ribbons are being processed into cable at elevated temperatures or during subsequent use of a completed cable, would cause the optical fibers to slightly buckle or bend. When the bending occurs as microscopic-sized bends (i.e., if the bending has components of spatial frequency between 0.1 and 10 per millimeter length of fiber), a partial loss in the optical signals being transmitted through the cable may occur. This well-known effect, called microbending loss, increases as the amount of tape shrinkage increases, since more tape shrinkage increases the amplitude of the microbending.
Heat-shrinkage of polyethylene terephthalte film has been greatly reduced in the past by a variety of heat-setting operations performed during manufacture of the film. However, even these stabilized films generally exhibit shrinkages of 0.4 to 0.6 percent or more in length upon exposure to 100.degree. C. for two hours, and of 3 percent or more after one hour at 175.degree. C.; and some previous films, including one that is highly stabilized against lengthwise shrinkage, exhibit high amounts of shrinkage in the transverse or cross direction. Furthermore, adhesive tapes made from the prior films tend to have even higher shrinkages, e.g., because of stresses introduced into the film during the adhesive-coating operation.
Shrinkages of 0.4 to 0.6 percent or more will produce undesirable microbending losses, and a temperature of 100.degree. C. that produces such shrinkage can be readily reached during manufacture of optical-fiber cable, thereby inherently flawing the cable. Shrinkages at 175.degree. C. are regarded as indicative of the long-term shrinkage that may occur during use of a completed cable, because shrinkages at that temperature are associated with fundamental molecular rearrangements, such as increased crystallinity, which may occur within a film over its useful life. A shrinkage of more than 3 percent after one hour at 175.degree. C. suggests a long-term instability that is undesirable in optical-fiber cables.
A rather thick film (0.0047 inch; 0.120 millimeter) commercially available from E. I. du Pont de Nemours and Company (Dimensionally Stabilized 500D film) has been reported to shrink 0.5 percent in length after againg for an unspecified time at 150.degree. C. However, films of such thickness would not be suited for optical-fiber cables due to the increased stiffness they would give a cable, the increased space they would occupy, and their higher cost. Furthermore, so far as known, a stable adhesive tape using such a film as a backing has not been supplied.
In summary, preparation of multifiber ribbons for optical transmission cables requires new thin polyethylene-terephthalate-backed adhesive tapes capable of apparently small, but significant and difficult-to-achieve improvements in dimensional stability.